ENERGY (THERMOCHEMISTRY) Ch 1.5, 6, 9.10, , 13.3

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Gwendolyn Quinn
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ENERGY (THERMOCHEMISTRY) Ch 1.5, 6, 9.10, 11.5-11.7, 13.

processes. Chemical Thermodynamics (with Thermochemistry) Addresses the Questions 1. Will a particular reaction occur? 2.

Energy (capacity to do work/cause heat transfer) (1.5, 6.

of potential and kinetic energy heat, q (E transferred between two bodies at different T) work, w (E used to cause motion against

1 ENERGY (THERMOCHEMISTRY) Ch 1.5, 6, 9.10, , 13.3 Thermochemistry Prediction and measurement of energy transfer, in the form of heat, that accompanies chemical and physical processes. Chemical Thermodynamics (with Thermochemistry) Addresses the Questions 1. Will a particular reaction occur? 2. If it does occur then a) what energy changes and transfers are involved? b) to what extent? Energy (capacity to do work/cause heat transfer) (1.5, 6.2) kinetic (motion) potential (position, chemical composition) energy can be transferred from one form to another interconversion of potential and kinetic energy heat, q (E transferred between two bodies at different T) work, w (E used to cause motion against an external force) interconversion of work and heat 1st Law of Thermodynamics Conservation of Energy: Energy can neither be created nor destroyed, only converted from one form to another (6.3). internal energy, E ΔE = q + w heat adsorbed by system and work done on system FIG I Energy Transfer Between a System and its Surroundings

- 2 - Definitions universe system surroundings system characterized with respect to transfers of matter/energy with surroundings open closed isolated surroundings boundaries (walls) characterized

. ability to predict REACTANTS initial state PRODUCTS final state prediction => state function => energy (ΔE?

2 - 2 - Definitions universe system surroundings system characterized with respect to transfers of matter/energy with surroundings open closed isolated surroundings boundaries (walls) characterized with respect to energy transfers in the form of heat adiabatic diathermal rigid/nonrigid WHY?. ability to predict REACTANTS initial state PRODUCTS final state prediction => state function => energy (ΔE?) => heat and work state function A State Function: Change in altitude depends only on the difference between initial and final values, not on the path. GOAL: correct state function can be used to predict the progress of a chemical reaction. Is Work the Desired State Function? (consider work in an ideal gas expansion) (6.4) FIG II - Gas Confined to a Cylinder with a Movable Piston FIG III - Calculating the Work = AΔh

4) FIG IV - Heat Transfer Between a Beaker of Water and its Surroundings Working with Heat Transfer- Calorimetry: Measurement of quantities of heat heat at constant

3 - 3 - EX 1. For the combustion of ethanol at room temperature in the open air CH 3 CH 2 OH(l) + O 2 (g) CO 2 (g) + H 2 O(l) What is the work done by the reaction? What is doing the work? What assumptions are you making? Is Heat the Desired State Function? (consider a beaker of hot or cold water) (6.4) FIG IV - Heat Transfer Between a Beaker of Water and its Surroundings Working with Heat Transfer- Calorimetry: Measurement of quantities of heat heat at constant pressure, q P heat at constant volume, q V exothermic, endothermic adiabatic (Greek, adiabatos impassable) thermal equilibrium heat capacity (rule of Dulong and Petit for molar heat capacities of metals)

4 - 4 - Heat Capacities of Some Substances (Table 6.4): c P = molar mass c s Substance Specific Heat Molar Mass Molar Heat Capacity c s (J K -1 g -1 ) (g mol -1 ) c P ( J K -1 mol -1 ) Hg(l) Al(s) Fe(s) Cu(s) He(g) H(g) H 2 (g) N 2 (g) CH 4 (g) CaCO 3 (s) CH 3 COOH(l) H 2 O(s) H 2 O(l) H 2 O(g) EX g of He absorbs kj of heat (at constant pressure) and the temperature increases by 15.0 K. What is C P? What is c s? What is c P?

5 - 5 - Calculations with Heat Transfer calorimeter EX g of Fe (c s = J K -1 g -1 ) at 400 K is put into 10.0 g of water (c s = 4.18 J K -1 g -1 ) at 290 K. a) Ignoring the heat capacity of the calorimeter, what is the final temperature? b) If the calorimeter has a heat capacity of 500 J K -1 what is the final temperature? EX 4. A styrofoam cup calorimeter contains 150 g of water. Generation of 1430 J of heat inside the calorimeter causes the temperature to increase by 1.93 C. What is C cal?

- 6 - Enthalpy (Greek, enthalpein to warm in), H (heat at constant pressure, q p ) (6.6-6.8, 13.

6 - 6 - Enthalpy (Greek, enthalpein to warm in), H (heat at constant pressure, q p ) ( , 13.3) enthalpy of reaction N 2 (g) + 3 H 2 (g) 2 NH 3 (g) per one mole of N 2 three moles of H 2 two moles of NH 3 2 N 2 (g) + 6 H 2 (g) 4 NH 3 (g) 2 NH 3 (g) N 2 (g) + 3 H 2 (g) ΔH = kj ΔH = -2(91.8) kj ΔH = kj enthalpy of phase changes FIG V- Energy Changes Accompanying Phase Changes (Greek, phasis- appearance) Common phase transitions involve a transfer of heat between a system and its surroundings at constant temperature and pressure vaporization l g (condensation g l) 11.6 fusion (melting) s l (freezing l s) 11.7 sublimation s g (deposition g s) Fusion I Internal Energy, E (heat at constant volume, q V ) (6.5) Relation between ΔH and ΔE EX 5. At atmospheric pressure and 25 C the detonation of nitroglycerin 4 C 3 H 5 (NO 3 ) 3 (l) 6 N 2 (g) + 10 H 2 O(g) + 12 CO 2 (g) + O 2 (g) releases 5720 kj of heat. What is the internal energy change for this reaction?

- 7 - Experimental Calorimetry heat at constant pressure, q P FIG VI - Coffee Cup Calorimeter heat at constant volume, q V FIG VII - An Adiabatic Bomb

The temperature of the calorimeter rises by 2.54 C. ΔE for the combustion of benzoic acid is -26.38 kj g -1. Determine the heat capacity of the calorimeter.

8) combining enthalpies of reaction enthalpy is a state function application to chemical reactions Hess s Law: The change in enthalpy for a stepwise

7 - 7 - Experimental Calorimetry heat at constant pressure, q P FIG VI - Coffee Cup Calorimeter heat at constant volume, q V FIG VII - An Adiabatic Bomb Calorimeter EX g of benzoic acid (C 6 H 5 COOH) are combusted in a constant volume bomb calorimeter. The temperature of the calorimeter rises by 2.54 C. ΔE for the combustion of benzoic acid is kj g -1. Determine the heat capacity of the calorimeter. (M = g/mol) Hess's Law - Extremely USEFUL (6.8) combining enthalpies of reaction enthalpy is a state function application to chemical reactions Hess s Law: The change in enthalpy for a stepwise process is the sum of the changes in enthalpy of the steps. A + 2B C C 2D EX 7. Find the enthalpy change that accompanies the combustion of graphite to carbon monoxide from: 1) C(s, gr) + O 2 (g) CO 2 (g) ΔH = kj mol -1 2) CO 2 (g) CO(g) + 1/2 O 2 (g) ΔH = kj mol -1 C(s, gr) + 1/2 O 2 (g) CO(g) ΔH =?

- 8 - FIG VIII- Enthalpy Diagrams for Chemical Reactions application to phase changes FIG IX

How much heat is required to lower the temperature of 2.5 mol of water from 130 C to -40 C?

02 kj mol -l a) H 2 O(g, 130 C) H 2 O(g, 100 C) : b) H 2 O(g, 100 C) H 2 O(l, 100 C) : c) H 2

8 - 8 - FIG VIII- Enthalpy Diagrams for Chemical Reactions application to phase changes FIG IX -Enthalpy Change upon Cooling 2.5 mol of Water from 130 C to -40 C EX 8. How much heat is required to lower the temperature of 2.5 mol of water from 130 C to -40 C? For water: c P (s), c P (l), c P (g) = 37.6, 75.38, 33.1 J mol -1 K -1, ΔH vap, ΔH fus = 40.7, 6.02 kj mol -l a) H 2 O(g, 130 C) H 2 O(g, 100 C) : b) H 2 O(g, 100 C) H 2 O(l, 100 C) : c) H 2 O(l, 100 C) H 2 O(l, 0 C) : d) H 2 O(l, 0 C) H 2 O(s, 0 C) : e) H 2 O(s, 0 C) H 2 O(s, -40 C) : overall: H 2 O(g, 130 C) H 2 O(s, -40 C):

9 - 9 - "Standard" Enthalpies (6.9) standard states stable form (allotrope) at P = 1 atm and specified temperature (usually T = 25 C) pure solid pure liquid gas ideal gas behavior 1 molar (1 M) solution ideal solution behavior standard enthalpy of formation, H f o, of a compound: enthalpy change for the reaction forming one mole of the compound from its elements in their standard states. H f o (element in standard state) = 0 EX 9. Find the enthalpy change that accompanies the combustion of methane to carbon dioxide from enthalpies of formation ( H f o in Appendix IIB) defining formation reaction H f o (kj mol -1 ) 1) C(s, gr) + 2 H 2 (g) CH 4 (g) ) O 2 (g) O 2 (g) 0 3) C(s, gr) + O 2 (g) CO 2 (g) ) H 2 (g) + 1/2 O 2 (g) H 2 O (l) long way: TARGET: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) short way: FOR ANY REACTION: H o rx = nn prod H o f (products) nn react H o f (reactants) => products - reactants

10 application to a) chemical reactions and b) phase changes EX 10. For the combustion of ethanol (unbalanced) CH 3 CH 2 OH(l) + O 2 (g) CO 2 (g) + H 2 O(l) a) Determine the standard enthalpy of the reaction given that H f o for CH 3 CH 2 OH(l), CO 2 (g), and H 2 O(l) are , , and , respectively. b) What is the standard enthalpy change when all reactants and products are gases? At 25 C, ΔH vap (H 2 O) = kj mol -1 and ΔH vap (CH 3 CH 2 OH) = kj mol -1. application to combustion reaction/ignoring the work term EX 11. Determine H f o of benzoic acid (C 6 H 5 COOH). ΔE o for the combustion of benzoic acid is kj g -1, H f o [CO 2 (g)] = kj mol -1 H f o [H 2 O(1)] = kj mol -1. (M = g/mol)

11 Average Bond Enthalpies (Chapter Bond Energies) Heat, at standard conditions, absorbed by a system to break chemical bonds in the gas phase. Bond Energy of a Homonuclear Diatomic bond enthalpy of chlorine gas (measured, 243 kj/mol): Cl 2 (g) 2 Cl(g) ΔH o =? ½ Cl 2 (g) Cl(g) o H f = = , close to 243 Bond Energy of Methane 1) atomization enthalpy of CH 4 (energy to break all bonds and form free atoms, atomize, in gas phase): CH 4 (g) C(g) + 4 H(g) ΔH o = 1663 kj/mol per CH, 1663/4 = 415 2) enthalpy of a C H bond in CH 4 (experimental results): CH 4 (g) CH 3 (g) + H(g) ΔH o = 439 kj/mol CH 3 (g) CH 2 (g) + H(g) ΔH o = 453 CH 2 (g) CH(g) + H(g) ΔH o = 425 CH(g) C(g) + H(g) ΔH o = 339 3) enthalpy of a C H bond: average = 1656/4 = 414 CH 4 (g) CH 3 (g) + H(g) ΔH o = 439 kj/mol C 2 H 6 (g) C 2 H 5 (g) + H(g) ΔH o = 410 CHF 3 (g) CF 3 (g) + H(g) ΔH o = 429 CHBr 3 (g) CBr 3 (g) + H(g) ΔH o = 377 above 4 within 8% of average Single Bonds Table 9-3. Average Bond Enthalpies H C N O F Cl Br I S Si H 436 C N O F Cl Br I S Si Multiple Bonds C=C 612 N=N 409 C=N 613 C=O 743 C C 812 N N 946 C N 890 C O 1046 O CO 2 799

12 FIG X. Estimate ΔH o for: CC1 2 F H 2 CH 2 Cl HF ) 3) 872 REACTANTS 2) ) 6) PRODUCTS 1) => reactants - products EX 12. Use average bond enthalpies to estimate ΔH o for 2 CH 2 = CH CH 3 (g) + 2 NH 3 (g) + 3 O 2 (g) 2 CH 2 = CH CN(g) + 6 H 2 O(g)

Chapter 8. Thermochemistry 강의개요. 8.1 Principles of Heat Flow. 2) Magnitude of Heat Flow. 1) State Properties. Basic concepts : study of heat flow

Chapter 8. Thermochemistry 강의개요. 8.1 Principles of Heat Flow. 2) Magnitude of Heat Flow. 1) State Properties. Basic concepts : study of heat flow 강의개요 Basic concepts : study of heat flow Chapter 8 Thermochemistry Calorimetry : experimental measurement of the magnitude and direction of heat flow Thermochemical Equations Copyright 2005 연세대학교이학계열일반화학및실험